Transference Hittorf method

Hittorf method phys chem A procedure for determining transference numbers in which one measures changes in the composition of the solution near the cathode and near the anode of an electrolytic cell, due to passage of a known amount of electricity. hi-dorf, meth-od ... 

The quantity [ zlLkf izfLJ is the transference number of the fcth ion as determined by the Hittorf method and, therefore, Equation (12.105) may be written as... 

Hittorf Method Experimental Procedure.—In Hittorf s original determination of transference numbers short, wide electrolysis tubes were used in order to reduce the electrical resistance, and porous partitions were inserted to prevent mixing by diffusion and convection. These partitions are liable to affect the results and so their use has been avoided in recent work, and other precautions have been taken to minimize errors due to mixing. Many types of apparatus have been devised for the determination of transference numbers by the Hittorf method. One form, which was favored by earlier investigators and is still widely used for ordinary laboratory purposes, consists of an H-shaped tube, as shown... 

Although the Hittorf method is simple in principle, accurate results are difficult to obtain it is almost impossible to avoid a certain amount of mixing as the result of diffusion, convection and vibration. Further, the concentration changes are relatively small and any attempt to increase them, by prolonged electrolysis or large currents, results in an enhancement of the sources of error just mentioned. In recent years, therefore, the Hittorf method for the determination of transference numbers has been largely displaced by the moving boundary method, to be described later. 

True and Apparent Transference Numbers.—The fundamental assumption of the Hittorf method for evaluating transference numbers from concentration changes is that the water remains stationary. There is ample evidence, however, that ions are solvated in solution and hence they carry water molecules with them in their migration through the electrolyte this will result in concentration changes which affect the measured or apparent transference number. Suppose that each cation and anion has associated with it and w- molecules of water, respectively let T+ and be the true transference numbers, i.e., the actual fraction of current carried by cations and anions, respectively. For the passage of one faraday of electricity the cations will carry w T+ moles of water in one direction and the anions will transport W-T- moles in the opposite direction there will consequently be a resultant transfer of... 

If the net amount of water (x) transported were known, it would thus be possible to evaluate the true and apparent transference numbers from the results obtained by the Hittorf method. 

It may be noted that the values obtained by the moving boundary method, like those given by the Hittorf method, are the so-called apparent transference numbers (p. 114), because the transport of water by the ions will affect the volume through which the boundary moves. It is the practice, however, to record observed transference numbers without applying any correction, since much uncertainty is attached to the determination of the transport of water during the passage of current. Further, in connection with the study of certain types of voltaic cell, it is the apparent" rather than the true" transference number that is involved (cf. p. 202). 

Influence of Temperature on Transference Numbers.—The extent of the variation of transference numbers with temperature will be evident from the data for the cations of a number of chlorides at a concentration of 0.01 N recorded in Table XXX these figures were obtained by the Hittorf method and, although they may be less accurate than those in Table XXIX, they are consistent among themselves. The transference... 

Much use of transference numbers has been made in the development of electrochemistry. The chief methods for their determination are (a) the Hittorf method, (6) die moving boundary method and (c) the electromotive force method. Of these the first two will be considered in this chapter. 

Transference Measurements by the Hittorf Method. The early transference measurements by the Hittorf method have been summarized by McBain, and by Noyes and Falk.5 In general this early work contained enough sources of error to make its use uncertain in studying the properties of salt solutions. However, recent researches by Jones... 

In spite of its simplicity accurate results are very difficult to get with the Hittorf method. The main difficulties are, first, the necessity for avoiding mixing of the electrode and middle portions during an electrolysis, which may take from sixteen to twenty-four hours, and secondly, the need for extremely accurate analyses of the solutions, since the method depends essentially on small differences between large quantities. For these reasons the more recent accurate data on transference numbers have been obtained, in greatest part, by means of the more complicated, but more speedily and accurately carried out, method of moving boundaries, which will be next described. 

The Hittorf transference number may be defined as the number of equivalents of a given ion constituent which, on passage of one faraday of electricity, cross a plane fixed with respect to the solvent, usually, of course, water. In a determination by the moving boundary method the position of a boundary is fixed with respect to the graduations of the tube. Hence, in order to obtain a value of a transference number comparable with that found by the Hittorf method, the motion of the water with respect to the tube must be computed. 

Although we have assumed that the moving boundary and the Hittorf methods, when correctly used, yield the same values of transference numbers, there has been no adequate test of this assumption until recently. The only measurements of transference numbers by the Hittorf method available for this comparison, in which modern technique has... 

Table V. Cation Transference Numbers at 25° of Aqueous Solutions of Electrolytes Determined by the Hittorf Method, Compared with the Results of Moving Boundary Determinations...

Here NaCl (ft) represents a solution of sodium chloride, the chemical potential of the solute of which is and fan is the number of equivalents per faraday transferred from the higher to the lower concentration during the reversible operation of the cell. It will be noted that the operation of cell (12) is, differentially and reversibly, the opposite of the Hittorf method for determining the Hittorf transference number fan as described in Chapter 4. If now a second cell is made as follows... 

Solution Molality of solution m Emf, volts per centimeter height, X10 ——Cation Transference Number. Gravity Hittorf method method Reference... 

Transference numbers are often measured by the Hittorf method as illustrated in this problem. Consider the three-compartment cell ... 

The moving-boundary method yields more accurate data on transference numbers than does the Hittorf method. Experimentally it is easier to handle. The difficulties lie in the establishment of a sharp boundary, the necessity of avoiding convection currents, and excessive heating by the current. However, once the boundary is established, the flow of current sharpens the boundary, making this a minor difficulty. The relative concentrations of the two solutes are important in maintaining a sharp boundary. The faster moving ion, M in this example, does not lead by more than a few atomic diameters, since a potential difference develops in such a sense as to slow it down in the steady state the two ions move with the same velocity, but M is always a little bit ahead of M. 

Around the turn of the century discrepancies were noted between transference numbers obtained by the Hittorf method and those obtained by the moving boundary (m.b.) method. Explanations proposed in terms of complex ions were clearly unsatisfactory. 

One important point to make is that the quantity accessed experimentally, in general, is the net transference number for a given species regardless of its exact speciation (6). Eor complex ions, if the Hittorf method (see below) is used to measure the transport of chloropalladate ions for example, by analysis of the amount of Pd accumulated on an electrode surface, one has no way of distinguishing between the transfer of [PdCy " and [PdClj] , both of which occur in solution. The existence of rapidly occurring equilibria, which will typically interconvert on time-scales much shorter than the measurement time-scale, means that the net transference of anionic palladium species of whatever form is... 

The classical methods of experimental transference number determination can be divided into three general groups. The first (the Hittorf method) is essentially an analytical approach, which relates changes in cell composition to the transference numbers of the electrolyte solution. The second group of methods relates the motion of the boundary separating zones of different composition to the transference numbers. The final approach relates the cell potential, which arises from the diffusion potential, to the transference number. Each of these methods is summarised, in turn, below. 

Figure 20.5 Cell used by Jones and Dole (8) for calculation of transference numbers by the Hittorf method. The solution compartment is further sub-divided into sub-compartments (see Figure 20.3), although the composition of each sub-compartment is initially identical.

The principal data on the transport phenomena in cryolite melts was discussed in the monograph Aluminium Electrolysis [2], Transference (transport) numbers are discussed also in the third edition of Aluminium Electrolysis [3]. The treatment is based on results published by Frank and Foster [4], Tual and Rolin [5, 6], and Dewing [7]. Frank and Foster investigated transport phenomena in cryolite-alumina melts by means of a radioactive tracer method. It was found that = 0.99. Tual and Rolin applied the classical Hittorf method. These authors also came to the conclusion that in neutral or basic electrolytes the transference number of the sodium cation is close to unity. With increasing acidity of the bath, the transport number of Na" " decreases. This is often explained by participation of the F ions in the conduction [2, 3]. Even in electrolytes with an excess of 7 mass% AIF3, the transference number of the sodium cation did not drop below Na L13A1F5 melt at 1030 K, Dewing [7] found that the transport number t+ = 0.957 0.08. 

Sterten et al. [8] reported transference numbers for Na+ based on emf measurements for CR = 2-3 [molar ratio nfNaFynfAlFj)] to be 0.96-0.99. It was assumed that fluoride anions carry the remainder of the current. In the present work we used the Hittorf method in a similar way as was done by Tual and Rolin [5, 6]. 

In the case of small ions, Hittorf transference cell measurements may be combined with conductivity data to give the mobility of the ion, that is, the velocity per unit potential gradient in solution, or its equivalent conductance. Alternatively, these may be measured more directly by the moving boundary method. 

Based on the general scenario provided above, the analytical method to determine transference or transport numbers has been devised and is carried out in an apparatus which can essentially be regarded as an improvement over the Hittorf apparatus. This consists of two vertical tubes connected together with a U-tube in the middle all three tubes are provided with stop-cocks at the bottom. The U-tube is also provided with stop-cocks at the top by closing these, the solutions in the cathode and anode limbs can be isolated. The silver anode is sealed in a glass tube as shown, and the cathode is a piece of freshly silvered silver foil. The apparatus is filled up with a standard solution of silver nitrate and a steady current of about 0.01 ampere is passed for 2-3 hours. In order to avoid the occurrence of too large a change in concentration it is necessary to pass the current only for a short duration. The... 

Unequal velocities of ions cause changes in concentrations in the proximity of electrodes. From these changes the transference numbers can be calculated provided the quantity of electricity passed through the electrolyte is known (Hittorf s method). 

The methods of measuring the velocity of electrokinetic motion are fully described in some of the reviews mentioned above. They include (for cataphoresis) various forms of U-tube in which the motion of the boundary of the suspension is observed, transference methods similar to Hittorf s transport number measurements in electrochemistry, and microscopic cells in which the motion of individual particles is watched, due allowance being made for the motion of the suspending fluid in the opposite direction to the particles. Sumner and Henry s device1 of fixing a sphere on a fibre and observing its deflexion in a horizontal electric field is very ingenious, and not so frequently mentioned as other methods. 

Different methods for the study of selective solvation have been developed [118, 120] conductance and Hittorf transference measurements [119], NMR measurements (especially the effect of solvent composition on the chemical shift of a nucleus in the solute) [106-109], and optical spectra measurements like IR absorption shifts [111] or UV/Vis absorption shifts of solvatochromic dyes in binary solvent mixtures [124, 249, 371]. Recently, the preferential solvation of ionic (tetralkylammonium salts) and neutral solutes (phenol, nitroanilines) has been studied particularly successfully by H NMR spectroscopy through the analysis of the relative intensities of intermolecular H NOESY cross-peaks [372]. 

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